All references, including any patents or patent applications, cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents forms part of the common general knowledge in the art, in New Zealand or in any other country.
Myostatin (MSTN) is a recently discovered member of the TGF beta super-family. Myostatin mRNA and protein have been shown to be expressed in skeletal muscle, heart and mammary gland. Targeted disruption of the myostatin gene in mice and a mutation in the third exon of myostatin gene in double-muscled Belgian Blue cattle, where a nonfunctional myostatin protein is expressed, leads to increased muscle mass. Hence myostatin has been shown to be a negative regulator of skeletal muscle growth.
The transforming growth factor-(TGF-β) superfamily of genes encode secreted factors that are important for regulating embryonic development and tissue homeostasis in adults. McPherron et al., (1997) described myostatin for the first time as being expressed specifically in developing and adult skeletal muscle and functioning, as discussed above, as a negative regulator of skeletal muscle mass in mice. Recent reports suggest that myostatin expression is not specific for skeletal muscle and that expression of myostatin is seen at least in heart and mammary gland, although studies reported in mouse, cattle and pigs have indicated that high levels of myostatin are detected specifically in developing and adult skeletal muscles (Kambadur et al., 1997; McPherron et al., 1997; Shaoquan et al 1998). Initially myostatin gene expression is detected in myogenic precursor cells of the myotome compartment of developing somites, and the expression is continued in adult axial and paraxial muscles (McPherron et al., 1997). Furthermore it was also shown that different axial and paraxial muscles expressed different levels of myostatin (Kambadur et al., 1997).
Myostatin null mice show a dramatic and widespread increase in skeletal muscle mass due to an increase in number of muscle fibres (hyperplasia) and thickness of fibres (hypertrophy) (McPherron et al., 1997). Subsequently, the present inventors (Kambadur et al., 1997) and others (McPherron and Lee, 1997; Grobet et al., 1997) reported that the Belgian Blue and Piedmontese breeds of cattle, which are characterized by an increase in muscle mass (double-muscling), have mutations in the myostatin coding sequence. This data suggests that somehow myostatin is a genetic determinant of skeletal muscle mass, and that myostatin is a negative regulator of muscle growth.
The molecular mechanism of action of myostatin is not known. The TGF beta proteins generally are synthesized as inactive precursor proteins, and at some point during the secretion the precursor protein is proteolytically processed and the processed mature protein is secreted. Two molecules of circulating processed protein together form a homodimer with “cysteine knot” structure, which elicits biological function by binding to its respective receptor. It would be desirable to understand the functions of myostatin at the molecular level in order to understand its biological action more fully, and to enable mimetics of both normal and mutated myostatin to be identified and used as therapeutic agents.
It is an object of the present invention to go some way towards achieving these goals, or at least to provide the public with a useful choice.